[jira] [Created] (AMQ-6396) JVM property org.apache.activemq.transport.nio.SelectorManager.maximumPoolSize has no impact

["Jason Baik (JIRA)" <ji...@apache.org>]

Jason Baik created AMQ-6396:
-------------------------------

             Summary: JVM property org.apache.activemq.transport.nio.SelectorManager.maximumPoolSize has no impact
                 Key: AMQ-6396
                 URL: https://issues.apache.org/jira/browse/AMQ-6396
             Project: ActiveMQ
          Issue Type: Improvement
          Components: Transport
    Affects Versions: 5.14.0
            Reporter: Jason Baik
            Priority: Minor


http://activemq.apache.org/nio-transport-reference.html makes it sound as if the number of threads in the SelectorManager's ThreadPoolExecutor can be capped with the JVM property: org.apache.activemq.transport.nio.SelectorManager.maximumPoolSize.

{code}
    protected ExecutorService createDefaultExecutor() {
        ThreadPoolExecutor rc = new ThreadPoolExecutor(getDefaultCorePoolSize(), getDefaultMaximumPoolSize(), getDefaultKeepAliveTime(), TimeUnit.SECONDS, new LinkedBlockingQueue<Runnable>(),
            new ThreadFactory() {

                private long i = 0;

                @Override
                public Thread newThread(Runnable runnable) {
                    Thread t = new Thread(runnable, "ActiveMQ NIO Worker " + (i++));
                    t.setDaemon(true);
                    return t;
                }
            }, new ThreadPoolExecutor.CallerRunsPolicy());

        return rc;
    }
{code}

However, this is not true since an unbounded LinkedBlockingQueue is being used with the the executor. The size of the thread pool will never grow past the core pool size as per ThreadPoolExecutor's Javadoc:

{quote}
If there are more than corePoolSize but less than maximumPoolSize threads running, a new thread will be created *only if the queue is full*.
{quote}

I think it's better not to include in the maximumPoolSize property in the documentation since it can confuse users into thinking that the pool size should be capped using the maximumPoolSize property, although in reality it's the corePoolSize property that does that.

Personally this confused me greatly and wasted me a lot of time while stress-testing the MQTT NIO connector for my company. The broker would constantly hit a live lock and spawn only 10 threads for the NIO transport, even if I set the maximumPoolSize to a larger number.



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